FIG. 1 is a block diagram of certain portions of a disk drive that are relevant to the present invention. With reference to FIG. 1, it is well known that magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer or head 81 that is located at the far end of a moveable actuator that is moved with a voice coil motor (VCM) 80. A sampled-data servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably move the actuator from track to track ("seeking") and to hold the actuator at a desired position ("track following"). It is unnecessary to provide a detailed discussion of the sampled servo control format, the servo control loop, or servo control systems in general because such systems are well known.
Of relevance to this invention, however, it can be seen in FIG. 1 that the control loop includes an N-bit digital-to-analog converter (DAC) 31' that is used to drive the VCM 80 via a suitable drive circuit 70'. In operation, the DAC 31' receives a coded value in the form of an N-bit demand and then produces an analog output signal having a magnitude corresponding to the value coded into the N-bit demand.
The particular system of FIG. 1 includes a controller 10 and a VCM driver chip 30' that are conventionally provided as separate elements. Other arrangements are possible, but it is important to note that, in order to accommodate the requisite power requirements, a coarser semiconductor fabrication technology is generally used to make the VCM driver chip 30' than is used to make the controller 10. As such, it is generally considered important to minimize the logic embedded within the VCM driver chip 30' because of costs associated with an increase in die size.
In general operation, the controller 10 outputs a sequence of N-bit demands to the DAC 31', over a serial interface 20, in accordance with host commands that are received from a host computer (not shown) and in accordance with sampled-servo information that was detected by a head 81 and communicated to the controller 10 via a channel 90.
Each N-bit demand in the sequence is coded (typically in the form of a binary code) to represent a demanded amount of current to flow through the actuator motor 80. The DAC 31' converts each N-bit demand into an analog output signal in the form of a voltage having a magnitude that corresponds to the N-bit demand. The drive circuit 70' suitably receives the analog output signal and controls the current in the coil of the actuator motor 80 accordingly.
The disk drive industry is highly competitive and manufacturers are continuously striving to increase areal density while reducing costs. One of the most common approaches to increasing areal density is to increase track pitch (tpi). In order to effectively track follow in the presence of more and more tightly packed tracks, however, it is necessary to provide finer and finer control of the VCM 80 with a higher resolution DAC. As an example, a 14-bit DAC is needed in order to provide the desired control resolution for contemporary areal density requirements.
Providing a 14-bit DAC on the VCM driver chip 30, however, uses a relatively large amount of die space as compared with a lower resolution DAC. Consuming more die space, of course, is contrary to the need to reduce costs.
There remains a need, therefore, to provide the higher resolution control needed to satisfy the market's ever increasing TPI requirements at a competitive cost.